In general, a casting die assembly comprising stationary and movable die halves of soft steel has been used to cast a plate grid for a lead-acid storage battery. A molten metal of lead or lead alloy is poured into a casting cavity or cavities formed between the closed die halves through an ingate and then cooled and solidified to produce a plate grid or grids. Since plate grids to be cast and in particular, the latitudinal members thereof are very thin and therefore, the cavities have a cross section sufficiently small to form such thin grids, no air vent is provided in the die assembly so that the molten metal cannot easily reach all the parts of the cavities. This causes the longitudinal and latitudinal members of the produced grids to discontinue.
In order to avoid such discontinuation of the members, there have been provided air escaping grooves in the casting surfaces of the die halves at the corners of the cavities to allow the air between the die halves to escape therein. However, only such grooves are not sufficient to avoid the discontinuation of the longitudinal and latitudinal members of the grids.
Alternatively or furthermore, many small relieves or bores have been provided in the casting surface of either of the die halves at the areas surrounded by cavity forming grooves, to allow the air to escape. Some of the bores which are far from the ingate, communicate through passageways with each other and also with the atmosphere, if any. However, in case the casting surfaces of the die halves are precisely or flatly finished, the air in its stream is insulated from the bores in the die halves so that it cannot reach them. If the casting surfaces of the die halves are roughly finished, then the gaps are formed between them so that the air can reach the bores, but a part of the molten metal is also intruded into the gaps with the result that undesirable fins would be formed on the produced grids. If such fins extend to the air escaping bores in the die halves, then the bores would be filled with the fins so that they lose their function to allow the air to escape. In addition, lead or lead alloy filled and solidified in the bores is left together with parts of the fins because they are difficult to be removed therefrom. This holds the die halves from being closely engaged, which tends to promote formation of the fins on the grids. Ideally, the air escaping bores are so constructed that a molten metal is no intruded into the bores, but that only air is intruded into them. However, since the bodies of the die halves are sufficiently thick to maintain their physical strength, the bores have been obliged to be formed by drilling and therefore, they could not avoid a large diameter, which allows a molten metal to be intruded into them.
Thus, such conventional casting die assembly tends to produce an undesirable plate grid having either fins or a discontinuation of longitudinal or latitudinal members of the grid.